This invention relates to satellite communication systems and, in particular, to communication links between non-stationary communication units of a satellite communication system.
In a typical satellite communication system, satellites may be required to communicate with other satellites to transfer data from a source node to a destination node. A source node or destination node may, for example, be a ground-based cellular telephone or a satellite within the satellite constellation. Communication between satellites may be performed by transmitting information from a satellite associated with a source node to a satellite associated with the destination node, or another intermediate satellite.
A direct communication link between two satellites is referred to herein as a cross-link. A cross-link is maintained where the communication antennas of both the source and the destination, or intermediate satellite(s), are pointed toward each other and data communication is occurring.
During each orbit, a non-geosynchronous satellite will travel in both an ascending (i.e. northbound) and a descending (i.e. southbound) direction whereby the direction changes as the satellites reach their maximum northern and southern latitudes. Where multiple, parallel orbital planes exist, satellites in a first orbital plane may travel in the same direction as satellites in an adjacent orbital plane, such as in an inclined orbit, or the satellites in the first orbital plane may travel in the opposite direction as satellites in an adjacent orbital plane (e.g., satellites in a first orbital plane are ascending and satellites in an adjacent orbital plane are descending), such as in a polar or an inclined orbit.
Typical satellite communication systems do not provide a method or apparatus for maintaining a cross-link with satellites in an adjacent orbital plane that are travelling in opposite directions. Because of this, a data packet intended for a destination satellite that is travelling in an opposite direction from the source satellite may not be transmitted directly to the destination satellite. Rather, the data packet must be transmitted through intermediate satellites travelling in the same plane, or a parallel orbital plane, until it reaches an intermediate satellite that is travelling in the same direction as the destination satellite. Then, the data packet may be transmitted, potentially through several intermediate satellites, to the destination satellite. However, each intermediate satellite that a data packet must be transferred through increases the amount of time it takes for the data packet to travel from its source to its destination. Therefore, in a communication system that transmits data packets containing voice data, multiple satellite transfers results in a noticeable delay in receipt of a voice signal.
Referring to U.S. Pat. No. 5,579,536, assigned to the same assignee as the subject application, there is disclosed a method and apparatus for establishing links between non-stationary communication units for a satellite constellation comprised of polar orbits, such as the Iridium(copyright) communication system manufactured by Motorola, Inc. The subject matter of the ""536 patent is incorporated by reference herein. The ""536 patent, however, only discloses a method and apparatus for cross-linking satellites between adjacent planes that are travelling in opposite directions and are within a xe2x80x9clink boundary corridorxe2x80x9d. The ""536 patent does not address a comprehensive partnering scheme for static or dynamic cross-links that is common to all satellites within the constellation, nor does it provide for the establishment of cross-links between satellites traveling in opposite direction and in non-adjacent orbital planes.
Therefore, what is needed is a method and apparatus for establishing a partnering scheme for a dynamic cross-link between satellites travelling in different directions and that are positioned within non-adjacent orbital planes such as is the case in an inclined orbital plane constellation. Such a partnering scheme with dynamic cross-links would minimize the number of satellite transfers that a data packet must undergo thereby enhancing system performance.